planner: simplify visited table tracking in collect_column_stats_usage #67912

Enhancement

pkg/planner/core/rule/collect_column_stats_usage.go currently keeps both visitedPhysTblIDs and visitedtbls.

In collectPredicateColumnsForDataSource(), both are populated with the same table ID (ds.TableInfo.ID), while visitedPhysTblIDs is already collected unconditionally.

Proposal

• reuse visitedPhysTblIDs for plan replayer table stats recording
• remove the redundant visitedtbls field
• clarify the visitedPhysTblIDs name/comment if needed, since the current code inserts table IDs rather than ds.PhysicalTableID

Benefit

Reduce duplicated state and make the collector easier to understand, with no intended behavior change.